What Is Terracotta Clay Made Of?

Terracotta clay, sometimes written as Terra cotta, is literally translated in Italian as 'fired earth'. It is also known as baked earth.

Essentially terracotta is a type of clay that has a few unique characteristics, in particular a red brown color that typifies its appearance in ceramics. It's widely used a material for tiles or pots for the garden, which is why we see so many pots take on this red brown color. It is also quite a porous clay, which again makes it ideal for garden pots. Unglazed, it is porous but with the addition of glaze it can be, and is used for roof tiles and other projects which require waterproofing.

Terracotta has been around for centuries. Until the 14th century, it was the only clay product used throughout history for any form of ceramics. The evidence of this is that terracotta has been found in historical architecture, including the famous terracotta army of Emperor Qin Shi Huang, believed to be from around 210 CE.

What is terracotta made of?

Terracotta clay can be formed in two ways:

1. It is dug up from the ground and then processed into terracotta clay. 
2. Manufactured by combining different raw materials together.

Terracotta from the ground

Terracotta clay forms underground over centuries. It is made from a mixture of rock and stone that has been worn to sediment by the natural elements, with the addition of other elements.

Typically when rock is weathered, it collects at its point of origin. This type of process, whereby material collects at a point of origin and turns to clay is called primary clay.

However, most rock sediment is moved by water and wind and collects at a different location.  On its journey, it picks up impurities and contaminants. This is known as secondary clay.  It contrasts with primary clay which is quite pure and uncontaminated.

Terracotta is known as a secondary clay because it contains other contaminants and elements, with one of the most prevalent being iron oxide. This creates the red color because when terracotta is fired in a kiln, the iron mixes with the oxygen to create the unique red brown characteristic with which terracotta is synonymous.

It is not always a uniform red however. Some terracottas are grey or a salmon pink color when unfired. However, most is red or orange-brown when it’s been fired.

The prevalence of iron within the clay does not just change the color of terracotta but also affects the temperature at which the clay can be fired at within the kiln.

Iron operates as a flux that lowers the temperature at which a materials will melt. Most clays contain elements like silica, which when heated to a high temperature will melt and seal the gaps between the clay particles, enabling the clay to become non-porous.

Because terracotta contains a significant amount of iron, it will melt at a lower temperature than other types of clay such as stoneware or porcelain. This leads to terracotta clay being more porous as only partial elements of the clay body melts and so some gaps are left.

What temperature can terracotta be fired at?

Typically, terracotta is fired between cone 04 and 03, a temperature range of between 1070c (1958F) to 1101c (2014F), a low fire clay.

Manufactured terracotta

In addition to terracotta being dug naturally, it can also be manufactured using various raw materials. This means that you can make terracotta to your exact specifications, making it less or more porous than you require.

An example recipe for terracotta is:

• Redart
• Talc
• AP Green
• Ball Clay

One issue that potters can find with fired terracotta pottery is that white spots can form on the finished ceramic. This is a build up of salt minerals and deposits. A way to avoid this is to add a touch of barium carbonate to your recipe.

What do terracotta's properties lend themselves to?

Terracotta's makeup means that it lends itself to a number of different functions. The ingredients of terracotta ensure that it is a flexible and easily manipulated clay.

Over the centuries, the use of terracotta has been applied to a number of different end products. It has been used by artists to create figurines and used by sculptors to create fine sculptures. It was also used in the Ancient Greek era as a way to decorate structures and for architectural purposes on buildings.

In modern times it has been used for tiles, bricks, planters and plant pots. Terracotta clay has a fairly low durability when compared to other clays but it is easily molded and porous which gives it unique and useful characteristics.

How can terracotta be waterproofed?

Many potters will want to ensure that their terracotta product is waterproof and non-porous. This can be achieved through adding glaze to the surface of the objects. A glazed terracotta bowl or plate can often be seen in Mediterranean or South American countries, where terracotta is a very popular clay to use.

The makeup of terracotta is perfect for creating art pieces that may also require glaze to ensure an extra layer of solidity and protection to the piece.

Summary

In summary of the title of this piece, 'What is terracotta made of?', it can be summarised as a clay that has a high iron content, and is fired at relatively low temperatures. 

This iron content also leads to the red appearance. However, terracotta clay can be engineered by adding some different minerals and elements, dependent on what its end purpose is. Terracotta is a really multipurpose clay and can be used for so many end uses, glazed or unglazed.

The simplest way to use terracotta clay is to get it straight from the ground, however there are numerous different recipes around which can be used to make bespoke versions suitable to your needs.